1. Field of the Invention
The invention relates to an apparatus and method for detecting an abnormal air-fuel ratio variation among cylinders and, more particularly, to an apparatus and method for detecting a relatively large variation in air-fuel ratio among cylinders of a multi-cylinder internal combustion engine.
2. Description of Related Art
Generally, in an internal combustion engine provided with an exhaust emission control system that uses a catalyst, it is necessary to control the mixture ratio, that is, the air-fuel ratio, of an air-fuel mixture to be burned in the internal combustion engine in order to purify toxic substances in exhaust gas using the catalyst with high efficiency. To control the air-fuel ratio, an air-fuel ratio sensor is provided in an exhaust passage of the internal combustion engine, and then feedback control is executed such that the air-fuel ratio detected by the air-fuel ratio sensor coincides with a predetermined target air-fuel ratio.
On the other hand, in a multi-cylinder internal combustion engine, air-fuel ratio control is usually executed over all the cylinders using the same control amount, so an actual air-fuel ratio may vary among the cylinders even when air-fuel ratio control is executed. At this time, if the variation is small, the variation may be absorbed by air-fuel ratio feedback control, and, in addition, toxic substances in exhaust gas may be purified by the catalyst. Therefore, such a small variation does not influence exhaust emissions.
However, for example, if a fuel injection system of part of the cylinders fails and, therefore, the air-fuel ratio varies by a large amount among the cylinders, exhaust emissions deteriorate. It is desirable that such a large air-fuel ratio variation that deteriorates exhaust emissions is detected as an abnormality. Particularly, in the case of an internal combustion engine for an automobile, it is required to detect an abnormal air-fuel ratio variation among the cylinders in an on-board state in order to prevent travel of a vehicle that emits deteriorated exhaust emissions. In recent years, there are moves to regulate travel of such a vehicle.
For example, an apparatus described in Japanese Patent Application Publication No. 2008-144639 (JP 2008-144639 A) detects an abnormal air-fuel ratio variation among cylinders on the basis of a fluctuation in the air-fuel ratio of an internal combustion engine. In addition, at the time of detecting the air-fuel ratio, the apparatus corrects an error of a detected air-fuel ratio on the basis of an engine rotational speed and an intake air flow rate.
An apparatus described in Japanese Patent Application Publication No. 2009-180171 (JP 2009-180171 A), which is configured to detect an abnormal air-fuel ratio variation among cylinders on the basis of a fluctuation in the air-fuel ratio of an internal combustion engine, changes an injection ratio between a plurality of fuel injection valves provided for each of the plurality of cylinders and then identifies which fuel injection valve causes the abnormal variation on the basis of an air-fuel ratio before changing the injection ratio and an air-fuel ratio after changing the injection ratio.
However, when the configuration described in JP 2008-144639 A is directly applied to an internal combustion engine having a plurality of fuel injection valves for each cylinder, it leads to erroneous detection due to a change in injection ratio. In addition, the configuration described in JP 2009-180171 A is able to determine whether there is an abnormality and to identify which fuel injection valve has an abnormality; however, there is also a possibility of erroneous detection due to a change in injection ratio.